Relatively high temperature heat sources are required for drawing high strength, low loss, high silica-content optical fiber from a preform or draw blank. Induction and resistance furnaces have generally been employed for drawing such silica fibers. Resistance furnaces, which typically are made of carbon, require an inert, protective atmosphere to prevent oxidation of the heating element.
An induction furnace conventionally includes a housing in which there is centrally disposed a tubular, yttria-stabilized zirconia susceptor surrounded by a cylindrical quartz beaker containing granular zirconia insulating material. The insulating material is commonly referred to as insulating grain or grog. An induction coil surrounding the beaker provides an alternating electromagnetic field that couples to the preheated zirconia susceptor, elevating the temperature thereof and forming a hot zone therein. A glass preform is introduced into the hot zone, a portion of the preform is reflowed, and optical fiber is drawn therefrom. Although such furnaces have been effective for drawing optical fiber, microscopic particles of zirconia (typically 1-10 .mu.m) migrate from the insulating grain and deposit on the preform or the fiber being drawn; these particles readily penetrate the surface due to its low viscosity. The resultant discontinuities in the fiber surface can constitute localized fracture centers which reduce the mechanical performance of the fiber, and, in particular, reduce its tensile strength.
Techniques used to reduce the migration of zirconia particles from the susceptor tube are described in U.S. Pat. Nos. 4,533,378 and 4,735,826. A thin coating of material compatible with the susceptor material is deposited on the inner surface of the susceptor to prevent the migration of particles from the zirconia grog to the preform. In one of the disclosed embodiments, the deposited material is the same material (e.g. silica) as the preform heated therein. However, if cracks of sufficient thickness occur in the susceptor, the migration of zirconia particles resumes. Apparently such cracks are too large to be filled with the coating material, whereby microscopic particles from the insulating zirconia grain surrounding the susceptor can be drawn therethrough and deposit on the preform and/or drawn fiber.
U.S. Pat. No. 4,450,333 teaches that the migration of zirconia particles originating at the insulating grain can be reduced by providing the induction furnace with a sleeve interposed between the insulating grain and the susceptor. The lower ends of the susceptor and sleeve are supported on a bottom surface of the beaker and form a joint therewith. Because of the manner of support, contaminating dust is likely to enter the furnace bore through this joint. Also, the susceptor and sleeve are capable of shifting laterally on the beaker support surface; this could cause inadvertent increases in the size of gaps between the susceptor and sleeve and the beaker base as well as misalignment of the furnace bore with the preform-fiber centerline.
Zirconia particles originating at the insulating grain can also enter the furnace bore between the susceptor and the base on which it is supported, or between the top of the susceptor and the end disc through which the preform extends into the furnace bore. U.S. Pat. No. 4,547,644 teaches a sealing arrangement for each end of the fiber drawing furnace, thereby minimizing the number of access paths for zirconia dust to the furnace bore. The furnace includes a tubular susceptor which is disposed centrally within a beaker and a sleeve which is disposed concentrically about the susceptor. The sleeve is spaced from the susceptor and is surrounded by insulating grain. The movement of contaminating particles from the insulating grain into the interior of the susceptor is reduced by annular, felt-like discs at the bottom and top of the beaker.
Notwithstanding the deployment of the aforementioned susceptor coatings and furnace equipment, zirconia particles from the insulating grain have been found to migrate to the preform and/or drawn fiber.
Accordingly, there is a need for eliminating from the furnace bore particles originating at the insulator that is disposed between the susceptor and high frequency coil. When considering a replacement for the standard zirconia grog insulator, its ability to withstand high, fiber drawing temperatures and its effect on power consumption must be taken into consideration. A furnace utilizing a non-grog insulator should consume little or no more power than a standard zirconia grog furnace.